1. Field of the Invention
The present invention relates to an electric double-layer capacitor and a method for producing the same.
2. Description of the Prior Art
As one of means for obtaining a capacitor having a large capacity, there is a method utilizing the production of an electric double-layer which is obtained by contacting activated carbon particles with an electrolytic solution as disclosed in the specification of U.S. Pat. No. 3,536,963.
FIG. 1 is a cross-section view showing a construction of a basic element of such electric double-layer capacitor (hereinafter called a unit cell). In an actual electric double-layer capacitor, since the withstand voltage of the unit cell is comparatively low as described hereinafter, unit cells are stacked so as to match working voltage, as shown in FIG. 2.
First the unit cell will now be described. The unit cell 1 can be produced as described below. A gasket 2, which is made of cylindrical and non-conductive rubber, is vertically divided by a porous separator 3, activated carbon paste electrodes 4, which are produced by kneading activated carbon particles and diluted sulfuric acid, are filled into upper and lower spaces inside the gasket cylinder, respectively, and further the top and bottom of the cylinder are closed up tightly by conductive separators. The porous separator 3 is ion-permeable and electron non-conductive, while the conductive separators 5 are electron conductive and ion-impermeable.
In the unit cell obtained in such a way, its withstand voltage is determined by the electrolysis voltage of diluted sulfuric acid and is low and about 1.2 V. A number of cells required to match the working voltage are stacked so as to form the stacked unit cells 6 as shown in FIG. 2. The stacked unit cells 6 are contained within a metal case 7 as shown in FIG. 13 so as to form an actual electric double-layer capacitor. It may be understood that the stacked unit cells 6 may be constituted by one unit cell according to the working voltage.
Next, the above mentioned stacked unit cells 6 and the completed electric double-layer capacitor will now be described with reference to FIG. 3.
In the electric double layer capacitor shown in FIG. 3, the stacked unit cells 6 have a charge holding function and is contained within the metal case 7.
The stacked unit cells 6 have, for example, a construction in which six basic cells 1 are stacked.
The metal case 7, in which the stacked unit cells 6 are contained, applies such pressure as described below to the top surface and the bottom surface of the stacked unit cells 6 through caulked portions 8 provided by caulking an opening end, a first electrode plate 9, an insulator case 10 and a second electrode plate 11.
In the stacked unit cells 6 obtained as described above, there exists internal resistance which consists of contact resistance between activated carbon particles in the activated carbon paste electrode 4 within the unit cells 1, contact resistance between respective unit cells 1 forming the stacked structure and the like.
The value of the internal resistance decreases when the pressure applied from the top and the bottom of the stacked unit cells 6 increases. Therefore, in order to suppress the internal resistance of the entire electric double layer capacitor, it is required to apply the pressure to the stacked unit cells 6 from the top and the bottom thereof. For the purpose of applying the pressure, the above-mentioned caulked portions 8 are provided. Such caulked portions are described in, for example, U.S. Pat. No. 4,394,713 and Japanese Utility Model Laid-open No. 174,338/87.
The value of the above-mentioned pressure is determined in consideration of the value of the internal resistance, pressure resisting capacity of the stacked unit cells 6 and so on, and the value is normally about 10 kg/cm.sup.2.
Upon operating the electric double-layer capacitor, voltage is applied between the first electrode plate 9 and the second electrode plate 11 from outside. In this case, the second electrode plate 11 is electrically connected to the top of the stacked unit cells 6. The first electrode plate 9 is electrically connected to the bottom of the stacked unit cells 6 through the caulked portions 8 of the metal case 7, the lateral wall and the bottom surface. Therefore, the voltage from outside is applied to the top and the bottom of the stacked unit cells 6. Consequently the electric double-layer capacitor performs function as a condenser.
By the insulator case 10, the outside surface of the stacked unit cells 6 is insulated from the inside wall of the metal case 7 and the first electrode plate 9 is insulated from the second electrode plate 11.
The opening of the electrode plate side of the metal case 7 is closed tightly by epoxy resin 13 so as to prevent a foreign matter from mingling from the outside or to prevent a chemical fluid from invading upon washing.
In order to prevent the electric double-layer capacitor from short circuiting with other electronic parts when the capacitor is packaged on a printed circuit board, the metal case 7 is covered with an insulator sleeve 12.
In the above-mentioned conventional electric double-layer capacitor since a metal case is used as an armor, the capacitor is complicated in the structure and a large number of assembly parts are required so as to cause it to function as a capacitor.
Thus an insulator case is required to insulate the unit cells from the metal case and to insulate the electrode plates from each other.
An insulator sleeve is required to prevent the electric double-layer capacitor and other electronic parts from short-circuiting with each other.
In order to prevent a foreign matter from mingling into the interior or to prevent a chemical fluid such as washing liquid from invading, it is required to close tightly the opening of the metal case with resin and the like.
As mentioned above, in the conventional electric double-layer capacitor, since the capacitor is complicated in its structure and requires numerous parts to be assembled, while it is accompanied by the complicated process of production, it was not possible easily to reduce the production cost.
Generally in the electronic components, it is desired to have various directions for leading out lead wires therefrom because of the large freedom at the time of packaging of the components. However, in the conventional electric double layer capacitor, when the direction for leading out the lead wire is tried to be changed, design changes of each of the above-mentioned numerous parts are required, and further large-scale changes of the production process such as changes of production equipments and jigs and tools are required. The direction for leading out the lead wire is practically fixed.
Therefore, when the electric double-layer capacitor was used in an actual electronic device, it was difficult to package the parts onto printed circuit boards under the improved package density because of small freedom of arrangement of the parts. Since surface packaging was difficult, automation of packaging was difficult and reduction of packaging cost was prevented.